The mobility of mobile stations (MS) between different cells in a cellular communication network in connected mode is primarily based on measurement reports sent by the MS to the serving cell. The network can configure an MS to send a measurement report (MR) on the uplink if a certain condition, also referred to as the triggering criterion, is satisfied.
An MS implementing 3GPP LTE technology can be configured to trigger a MR based on measurement of reference signal receive power (RSRP) or reference signal receive quality (RSRQ). An MS may also be referred to as a UE. Based on the MR sent by the MS, the network can detect an event when the MS must be handed-over from one cell to another. Typical scenarios under which a handover occurs include the coverage-limited case when, for example, the MS is at the cell-edge. Another scenario under which handover occurs is in load balancing cases when the serving cell is heavily loaded.
Another instance under which handover occurs is when the MS suddenly encounters a coverage hole. For example, when the MS turns around the corner of a building in an urban micro-cell deployment, suddenly the serving cell power may drop steeply and a neighbor cell which was hitherto invisible becomes detectable. Irrespective of which mode the LTE UE is operating in (non-DRX or DRX) the base-station does not know the exact instant a neighbor cell becomes detectable. This neighbor cell could either be on the same carrier (intra-frequency) or on a different carrier (inter-frequency).
The network typically has the option of controlling the Layer 3 (L3) filter by configuring suitable time to trigger (TTT) and L3 coefficient (k). But these offer limited help in emergency handovers situations. In emergency handovers, the network might be required to handover the UE after one measurement report and in this scenario, the accuracy of L3 filtered RSRP/RSRQ measurements (sent after ascertaining that the triggering criterion holds for TTT amount of time) affects the handover performance. One can envision the following scenarios. Because of the inaccuracy in the L3 filtered measurement, the MS can fail to trigger a measurement report resulting in a dropped connection. Such a scenario may be characterized as missed-reporting. In another scenario, one neighbor cell can get falsely or incorrectly ranked higher than another neighbor cell (stronger than the first neighbor cell) leading to either handover failure or handover inefficiency. Such a scenario may be characterized as false-reporting.
L3 filtering for RSRP/RSRQ measurements in E-UTRA is currently defined by the following text in TS 36.331 version 8.3.0, section 5.5.2.8 as follows. If the IE QuantityConfig is received the UE shall, depending on the measurement quantity, apply filtering of the measurements for that measurement quantity according to the formula below. This filtering shall be performed by the UE before UE event evaluation. The UE shall depending on the reporting quantity also filter the measurements reported in the IE MeasuredResults. The filtering shall be performed according to the following formula: Fn=(1−a)·Fn−1+a·Mn. The variables in the formula are defined as follows: Fn is the updated filtered measurement result; Fn−1 is the old filtered measurement result; Mn is the latest received measurement result from physical layer measurements, the unit used for Mn is the same unit as the reported unit in the MeasurementReport message or the unit used in the event evaluation; a=1/2(k/4), where k is the parameter received in the filterCoefficent field of the IE QuantityConfig. NOTE: if k is set to 0 that will mean no layer 3 filtering. In order to initialize the averaging filter, F0 is set to M1 when the first measurement result from the physical layer measurement is received. The physical layer measurement results are sampled once every measurement period. Both the measurement period and the accuracy for a certain measurement are defined in 3GPP TS 36.133, Requirements in support of Radio Resource Management. Layer 3 filtering is applicable to all UE measurement quantities listed in 3GPP TS 36.331, Radio Resource Control. The layer 3 filtering shall be performed in the same domain as the measurement or reporting is done, i.e., logarithmic filtering for logarithmic measurements, etc. There shall only be one layer 3 filter per measurement quantity.
In addition to the filter coefficient a, there is a time to trigger (TTT) quantity that is signaled in the RRC QuantityConfig and ReportConfig messages, respectively. The above L3 filter is an infinite impulse response (IIR) filter in the log-domain, denoted as “log-IIR filter” in what follows. For a typical TTT configuration (˜100 ms), even in non-DRX mode, one measurement report sent by the MS is based on 2 or 3 Layer 1 (L1) measurements. We further note the following points. The TTT criterion checks that the L3 evaluation is persistent over a window of time. Even in the non-DRX case, the number of successive L3 evaluations that the TTT checks for is quite small (=3-5, for TTT=100-200 ms). In the DRX case, this number could be smaller (<=2 for DRX>=160 ms). It is easy to show that log-IIR filter results in a biased estimate of the RSRP/RSRQ levels (for example, even in static channels). When there is insufficient L3 averaging, the bias and the variance in the L3 filtered quantity can lead to large variations of the estimate around the nominal. The currently defined L3 filter does not provide any noise averaging when dealing with so few samples. Further, it can lead to noise enhancement in time-correlated interference.
When, for example, the user turns a corner resulting in a change in signal levels, the base station does not know the exact instant the UE is able to detect a neighbor cell. This applies to both intra-frequency and inter-frequency cell identification in both non-DRX and DRX modes. The base station therefore does not have a mechanism to send a RRC measurement reconfiguration message to change the L3 coefficient/TTT based on the situation. For example, it cannot dynamically program a larger TTT for the case when a cell is newly detected at the “right” instant.
The various aspects, features and advantages of the disclosure will become more fully apparent to those with ordinary skill in the art on consideration of the following Detailed Description and the accompanying drawings. The drawings have been simplified for clarity and are not necessarily drawn to scale.